As photosensitive resin original sheets for flexographic printing, those to be developed by organic solvents had been known. However, in such conventional photosensitive resin original sheets had safety problems for a human body and the environment such as toxicity, flammability and the like.
Then, photosensitive resin compositions for photosensitive resin original sheets which can be developed with an aqueous developer have been proposed as a substitute for such conventional photosensitive resin original sheets. For example, JP-A 63-8648 discloses a photosensitive resin composition in which hard and finely divided organic particles are included to improve properties of the resultant printing matrix such as mechanical strength, impact resilience and the like. JP-A 2-175702, JP-A 3-228060, JP-A 4-293907, JP-A 4-293909, JP-A 4-294353, JP-A 4-340968, JP-A 5-32743, JP-A 5-150451 and JP-A 5-204139 disclose photosensitive resin compositions in which finely divided crosslinkable resin particles are included to provide developability with an aqueous developer and aqueous ink resistance and to improve printability. JP-A 59-36731 discloses a photosensitive resin composition in which ink receptivity of the resultant printing matrix is improved by providing two-phase structure to the composition, whose continuous phase contains a diazo compound and a bichromate and whose dispersed phase contains particles having a particle size of 10 .mu.m or less.
However, since such a photosensitive resin composition has multiple phase structure, irradiated actuating light does not go straight through the composition, but it is refracted at the interfaces between phases and scattered in the resin. When such a photosensitive resin composition is used as an original printing sheet, the imaged part of a relief obtained by developing is thicker than its original image on a negative film, or the depth of a non-imaged part is smaller due to light scattering upon light exposure for imaging. Then, when printing is carried out by using such a relief, it is likely that the image of the resultant printed matter is thicker than that of the original image, dot parts darken (dot gain), and printing quality is deteriorated because too much ink is adhered to a non-imaged part. Since these tend to be accelerated by excess light exposure, attention should be given to avoid excess light exposure. However, for reproducing a minute or fine image as a relief after developing, sufficient light exposure is required. That is, the range of proper light exposure time (light exposure latitude) of a photosensitive resin printing original sheet having multiple phase structure is apt to be narrower.